1. Technical Field
The present invention relates to a fluid transmission device that transmits power between a first rotating element and a second rotating element by kinetic energy of a fluid and, specifically, to a fluid transmission device including a lock-up clutch.
2. Related Art
In the related art, an example of a fluid transmission device that transmits power between a first rotating element and a second rotating element by kinetic energy of a fluid is disclosed in Japanese Unexamined Patent Application Publication (JP-A) No. 2014-141987. The fluid transmission device described in JP-A-2014-141987 includes a front cover that receives power from an engine as the first rotating element, an impeller fixed to the front cover, a turbine hub that is coupled to an input shaft of a transmission as the second rotating element, a turbine shell fixed to the turbine hub, and a stator disposed between the impeller and the turbine shell.
The fluid transmission device is provided with a lock-up clutch. The lock-up clutch includes a piston mounted on the turbine hub as the second rotating element, a first plate fixed to the turbine hub, and a first torsion spring that transmits power between the piston and the first plate. A dynamic damper is mounted on the first plate. The dynamic damper includes a hub flange mounted on the first plate, a second torsion spring disposed between the hub flange and the first plate, and an inertia member fixed to an outer peripheral end of the hub flange.
According to the fluid transmission device described in JP-A No. 2014-141987, in a state in which the front cover and the impeller are rotating, hydraulic oil flows from the impeller to the turbine, and power is transmitted from the impeller to the turbine via the hydraulic oil. The power transmitted to the turbine is transmitted to the input shaft via the turbine hub. When the number of rotations of the input shaft is smaller than a certain value, the lock-up clutch is released.
When the number of rotations of the input shaft reaches the certain value, a piston moves toward the engine side by a hydraulic pressure, and a friction member is pressed against the front cover. In other words, the lock-up clutch is engaged, the piston rotates integrally with the front cover, and the power is transmitted from the front cover to the turbine hub via the piston and the first torsion spring.
When an engine speed is lowered, an amount of variation caused by the engine speed is increased by variations of combustion. According to the fluid transmission device described in JP-A No. 2014-141987, when the engine speed gets close to a specific value, the dynamic damper activates to restrain variations in the state of rotation of the turbine.
However, according to the fluid transmission device described in JP-A No. 2014-141987, the mass of the dynamic damper acts on the second rotating element also when the lock-up clutch is OFF. Therefore, an increase in rotational speed of the second rotating element may be disadvantageously impaired with the lock-up clutch released.